1. Field of the Invention
The present invention relates to an electric power steering apparatus using an electric motor as a power source for assisting steering power.
2. Description of Related Art
An electric power steering apparatus for a vehicle, for example, detects a steering torque applied to an input shaft by a relative angular displacement quantity between the input shaft connected with a steering wheel and an output shaft coaxially connected with the input shaft via a torsion bar, and drives an electric motor for assisting steering based on the detected torque. Then, by transmitting a rotating force of the electric motor to steering means via a pair of gears, an operation of the steering means in accordance with rotation of the steering wheel is assisted by rotation of the electric motor to reduce a burden of labor of a driver for steering.
In a case where a pair of gears is used as described above, due to an occurrence of backlash in a meshing portion of the pair of gears, reduction of meshing noise caused by the backlash must be sought.
As means for reducing meshing noise caused by the backlash, generally providing an adjustment mechanism for adjusting a distance between centers of rotation of a pair of gears, or enhancing dimensional accuracy of gear teeth of the pair of gears have conventionally been attempted. However, since such means make a structure more complicated and lower working efficiency of machining and assembly, remedial measures have been demanded.
To reduce meshing noise caused by the backlash, for example, using lubricating fats and oils having an elastic function has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2005-263989). According to this prior art, direct contact of tooth surfaces of the meshing portion is prevented by applying lubricating fats and oils to a pair of gears and causing the lubricating fats and oils to be present between tooth surfaces.
Incidentally, to reduce meshing noise using lubricating fats and oils, the lubricating fats and oils must always be present between tooth surfaces in the meshing portion. Thus, fluidity must be improved by increasing a consistency value of the lubricating fats and oils to increase lubricity of the lubricating fats and oils. However, since lubricating fats and oils tend to adhere to inner surfaces of a housing where a pair of gears are housed and supported as fluidity of the lubricating fats and oils increases, a problem that a quantity of lubricating fats and oils circulating with rotation of the pair of gears decrease may occur.